1. Field of the Invention
The present invention is directed to vibratory separators, screen-supporting baskets for them, and, in certain particular embodiments, to shale shakers for treating drilling fluid, screen-supporting baskets for them, screens for such separators and shakers and methods of using such screens, shakers, baskets, and separators.
2. Description of Related Art
Vibratory separators are used in a wide variety of industries to separate materials such as liquids from solids or solids from solids. Typically such separators have a basket mounted in or over a receiving receptacle and vibrating apparatus for vibrating the basket. One or more screens is mounted in the basket. Material to be treated is introduced to the screen(s). Separated material (e.g. liquid and/or smaller solids) flows through the screen(s) into the lower receptacle and separated larger solids (with or without liquid) move down and off the screen(s).
The need for solids control in drilling fluid or “mud” used in hydrocarbon well drilling is well known in the prior art. Drilling mud, typically a mixture of clay and water and various additives, is pumped down through a hollow drill string (pipe, drill collar, bit, etc.) into a well being drilled and exits through holes in a drillbit. The mud picks up cuttings (rock) and other solids from the well and carries them upwardly away from the bit and out of the well in a space between the well walls and the drill string. At the top of the well, the solids-laden mud is discharged over a shale shaker, a device which typically has one or a series of screens arranged in tiered or flat disposition with respect to each other. The prior art discloses a wide variety of vibrating screens, devices which use them, shale shakers, and screens for shale shakers. The screens catch and remove solids from the mud as the mud passes through them. If drilled solids are not removed from the mud used during the drilling operation, recirculation of the drilled solids can create weight, viscosity, and gel problems in the mud, as well as increasing wear on mud pumps and other mechanical equipment used for drilling.
Typically, the screens used with shale shakers are emplaced in a generally horizontal fashion on a generally horizontal bed or support within a basket in the shaker. The screens themselves may be flat or nearly flat (i.e. substantially two-dimensional); or, due to corrugated, depressed, or raised surfaces, are three-dimensional. The shaker imparts a rapidly reciprocating motion to the screen. Material from which particles are to be separated is poured onto a back end of the vibrating screen. The material generally flows toward the front end of the screen. Large particles are unable to move through the screen, remain on top of the screen, and move toward the front of the screen where they are collected. The smaller particles and fluid flow through the screen and collect in a bed, receptacle, or pan beneath the screen.
Referring now to FIG. 1, a prior art shale shaker 1 has a screen 2 (with screen or screening cloth or mesh as desired) mounted on vibratable screen mounting apparatus or “basket” 3. The screen 2 may be any known screen or screens. The basket 3 is mounted on springs 4 (only two shown; two as shown are on the opposite side) which are supported from a frame 6. The basket 3 is vibrated by a motor 5 and interconnected vibrating apparatus 8 which is mounted on the basket 3 for vibrating the basket and the screens. Elevator apparatus 7 provides for raising and lowering of the basket end. Typically the basket will be in a “climb the hill” position (see FIG. 4) so that a pool 9 of liquid is maintained at one end within the basket.
FIG. 2 discloses one example of a typical prior art shaker system (e.g. as shown in U.S. Pat. No. 5,190,645). The system A has a shale shaker K having a screen or screens S. The screen(s) S are mounted in a typical shaker basket B and one or more vibrators V (any known suitable shaker vibrator) vibrate the basket B and hence the screen(s) S. The other components of the system A are as described in U.S. Pat. No. 5,190,645 incorporated fully herein for all purposes.
FIG. 3 shows schematically a prior art system C with a Mud Box (Back tank, Possum Belly) to distribute the flow to a screening surface. The screens are mounted in a Deck (Basket) which is vibrated to assist the throughput of mud and movement of separated solids. The deck rests on Vibration Isolation Members, such as helical springs or rubber mounts. The vibration isolation members rest on the support member, which is also used to divert flow as desired, and is called a Bed. There are many deck, basket configurations used depending on the design criteria. The deck, basket, may be flat, horizontal, or mounted at a slope.
On sloped deck units (e.g. cascade or parallel flow), the screens may be continuous with one screen covering the entire deck length, or have a divided deck which has more than one screen used to cover the screening surface, or with individual screens mounted at different slopes. On multiple deck units, more than one screen layer may be employed. In a two or three deck unit, the mud passes through one screen before flowing through the second.
The motion of the shaker controls the efficiency of separation, rate of travel of cuttings on the screen, solids separation capacity and fluid capacity. The shape and axial direction of the vibration motion along the deck is controlled by the position of the vibrator relative to the deck and the direction of rotation of the vibrator.
Shale shakers have capacity limits. Exceeding a capacity limit means excessive mud will be discharged over the ends along with the solids. Capacity limits are typically defined for non-blinded screens. Capacity limits of a shale shaker include:                1. The solids capacity limit is the maximum amount of solids that a device will remove; and        2. The liquid limit—the maximum GPM capacity for various drilling muds.        
The solids capacity limit is encountered when drilling soft, gummy formations. Usually the liquid limit controls the minimum size screen that can be used for a given circulation rate. The thicker the mud, the lower the GPM capacity. Open area of the screens and usable area of the screening deck influence this limit. The solids capacity and/or liquid capacity varies with different shaker designs. In practice, the smallest screen size that can be employed without flooding a unit is used.
There is a need for an efficient vibratory separator system which is efficient and cost-effective with increased efficiency, and screen throughput.